Various shielding and exhaust door configurations have been developed in the prior art for the purpose of minimizing the exposure of the maintenance personnel to potentially harmful radiation during maintenance operations. However, before the design objectives and shortcomings associated with such doors can be appreciated, some understanding of the general structure and operation of steam generators is necessary.
Nuclear steam generators are comprised of three principle parts, including a bowl-shaped primary side, a cylindrically shaped secondary side disposed over the primary side, and a tube sheet disposed between the primary and the secondary sides. The secondary side contains a bundle of U-shaped heat exchanger tubes, the ends of which are mounted within the tube sheet. The tube sheet and the U-shaped heat exchanger tubes hydraulically isolate the primary side from the secondary side of the steam generator while thermally connecting them together, so that heat from radioactive water circulating through the primary side may be transferred to non-radioactive water present in the secondary side in order to generate non-radioactive steam. The bowl-shaped vessel of the primary side is known as the channel head of the steam generator.
During the life of the steam generator, the heat exchanger tubes channel head may become corroded or damaged, thereby necessitating various inspections and repairs. These repairs are accomplished by a succession of tools which are remotely manipulated within the channel head. Access to the interior of the channel head is gained through manways on both the inlet and the outlet sides of the channel head. Each of the manways is circular in shape, and is surrounded by circumferential flange. When the steam generator is in operation, a hatch cover is sealingly attached over the manway flange by a plurality of uniformly spaced bolts that screw into bolt holes present in the flange. When repairs are necessary, the steam generator is taken offline and drained, and the hatch cover over each of the manways is removed to provide access for the repair tools. Because of the exposure of the channel head and the heat exchanger tubes to radioactive water, these components have likewise become radioactive. Consequently, radiation "shines" out of the manways after their respective hatch covers are removed. To reduce the amount of radiation exposure that the maintenance personnel receive from the radiation emitted through the manways, a set of lead-lined radiation reducing doors can be provided on both the inlet and the outlet manways.
In one prior art design, both a radiation reducing entry door and a ventilation door are provided for both the inlet and the outlet manways. Each of these doors includes a flange mounting assemblY that is detachably mountable to the bolt holes present in the flange which circumscribes the manways. The mounting assembly of each door consists of a half flange with mounting bolts large enough to be easily grasped by maintenance personnel wearing rubber gloves. These mounting bolts are suspended from the flanges by flexible cables so that the mounting bolts are easily accessible by the maintenance personnel when the time comes to manually screw them into the bolt holes of the manway flange. Each of the mounting assemblies also includes a hinge bracket detachably connectable by hinge pins to either the entry door or the ventilation door in order to obviate the removal and replacement of the mounting assemblies when it is desired to switch the ventilation and entry doors.
The radiation reducing entry door of this prior art design is semi-circular in configuration and weighs approximately forty pounds. While it is possible for one person to lift the door into position on the hinged bracket, such an operation requires the person to stand directly in front of the manway opening hence directly in the shine of radiation emanating therefrom. Since both the inlet manway and the outlet manway extend downwardly from the exterior wall of the channel head at an angle of 45 degrees relative to the horizontal, the entry door must be locked in both the open and closed positions since gravity tends to pull the door away from the manway. To lock the door in the fully opened condition, a ball lock pin must be inserted in aligned holes present in the hinged bracket and the door brackets. Again, this requires maintenance personnel to stand directly in the shine of the open manway to lift and position the entry door during insertion of the ball lock pin. Due to the heavy weight of the door, it is difficult for one person to swing the door to the completely open position and lock it in place. Therefore, two persons are required for this operation and each of these persons will be subjected to radiation emitted through the open manway. Likewise, two persons are required to lock the doors in the closed condition.
Similarly, the ventilation door of the prior art is provided with a hook shaped bracket which, during mounting, must be positioned over a bar extending between the hinged brackets on the top of the mounting assembly. In practice, this door weighs approximately seventy pounds and requires two persons to position it for mounting. Once this door is mounted on the hinged brackets, the door is pushed up against the manway flange and secured at its lower portion tightly against the manway flange by mounting bolts having large washers. The ventilation door includes a central opening having an eight inch diameter exhaust nozzle that extends away from its outer wall, while the inner wall is covered by a lead plate that is spaced from the opening to provide radiation shielding without interfering with the air flow through the nozzle.
Once these doors are in place, access may be gained through the lower portion of the inlet manway with radiation exposure being limited by the lead shielding in the entry door. Additionally, a vacuum is drawn through the nozzle in the ventilation door to draw air through the steam generator and out the ventilation door to remove airborne radioactive particulates from the interior of the steam generator. The lower portion of the inlet manway is adapted to receive servicing devices such as the SM-10 robotic arm supplied by Zetec of Isaquah, Washington as modified by the Westinghouse Electric Corporation. Often these servicing devices include a number of secondary manipulating devices. These secondary devices, such as the D-4 probe pusher manufactured by Echoram Technology, a Westinghouse subsidiary, are mounted on the trunk of the SM-10 robotic arm at the manway opening. When so mounted, the probe pusher extends upwardly from the trunk of the SM-10 and blocks the hinged movement of the entrance door. This requires the D-4 probe pusher to be removed each time the entrance door is required to be opened or removed, which in turn exposes the maintenance personnel to additional radiation.
Since the above-mentioned prior art entry door and ventilation door require the operator to stand directly in front of the open manway during the positioning, mounting, opening and closing of the doors, the operator is subjected to substantial amounts of potentially harmful radiation. Furthermore, these doors are formed of a laminate of lead shielding riveted between two sheets of stainless steel which is easily damaged from mechanical shock which might occur if the doors are dropped, and which also provides crevices for radioactive particles to accumulate. Finally, the size and weight of these doors makes this difficult to position and mount by a single person. Clearly, the need has arisen for entry and ventilation doors for manways which may be easily positioned and mounted on the manway flange by a single person, and that are easily opened and closed by a single person without subjecting that person to radiation exposure. Ideally, such doors should withstand mechanical shock without damage, be easily decontaminated, and easily interchanged between the inlet manway and the outlet manway. Finally, such doors should not mechanically interfere with the operation of standard maintenance tooling.